In https://mxtoolbox.com -> domain health test, we have this issue:
PROBLEM DETAILS
SMTP Banner Check
What you see when your domain has this problem
LISTED SMTP Banner Check
About SMTP Banner Check
The SMTP banner issued by your email server did not contain the hostname we resolved for your server’s IP address.
Additional Information
Email servers answer connections on port 25 with a string of text called an SMTP Banner whose purpose is to announce the server and any information that the administrator would like to convey to the world.
It is best practice to put the name of your server in your SMTP banner so that anybody who connects via your IP Address has a clue as to who they are talking to. You will get this warning if the name you present yourself as is not in the same domain as the hostname we get when we perform a PTR lookup on your IP Address.
For a time many servers would "mask" their SMTP banners by replacing the characters with asterisks to anybody outside of their network. The logic behind this was often that you did not want to broadcast any information about your network to people outside for fear of giving them any information that might help them in an attack against your server. The benefits of doing this are minimal and many servers perform a banner check as part of spam mitigation, so it has a negative cost associated with the practice. Our tool will issue a warning if your banner is masked.
Some receiving mail servers may use a mismatched or masked banner as an indication of a possible spam source in a scoring system, but most will not reject incoming mail solely on this basis.
If you do not have a PTR record, or your record does not match your hostname, we recommend that you contact your ISP and ask them to setup a reverse (PTR) record that matches the hostname of your mail server.
even though we've got DNS set as recommended in this thread.
What needs to be done?
id 8318 opcode QUERY rcode NOERROR flags QR RD RA ;QUESTION luzanky.cz. IN MX ;ANSWER luzanky.cz. 359 IN MX 5 alt1.aspmx.l.google.com. luzanky.cz. 359 IN MX 5 alt2.aspmx.l.google.com. luzanky.cz. 359 IN MX 10 alt4.aspmx.l.google.com. luzanky.cz. 359 IN MX 10 alt3.aspmx.l.google.com. luzanky.cz. 359 IN MX 1 aspmx.l.google.com. ;AUTHORITY ;ADDITIONAL